In a diamond, electron spins in a particular kind of colour centre, a nitrogen vacancy centre, can be polarised optically independent of ambient temperature. T Staudacher et al in “Nuclear Magnetic Resonance Spectroscopy on a (5 nanometre)3 Sample Volume”, Science (2013) demonstrate a scheme for sensing nuclear spin ensembles 5 nm (nanometres) from the diamond surface using shallow (close to surface) nitrogen vacancy centres. The authors note that a scheme for polarising the external nuclear spins can be developed, and might operate probably on a similar timescale to the detection. However, the authors have not provided any details as to how such a scheme could be achieved. Moreover, it appears that no clear scheme was thought of, as it currently appears that producing such a high polarisation rate with the proposed setup is not feasible, especially for small molecules, where the effect of molecular movement needs to be taken into account. In addition, the authors have only considered molecules in a (5 nm)3 radius.
In “Dynamic Nuclear Spin Polarization of Liquids and Gases in Contact with Nanostructured Diamond”, Nano Letters (2014) D Abrams et al show simulative results of polarisation of nuclear spins in a liquid in contact with a diamond surface filled with shallow nitrogen vacancy centres. The scheme uses direct polarisation of nuclear spins adsorbed on the surface of the diamond via the nitrogen vacancy (NV) centres and report a 0.2% to 2% polarisation of the external nuclear spins in a very small volume in a few seconds; the volume is a liquid layer of 0.5 μm (micrometres) thickness and 50 μm in length and width. Yet, the polarisation rate achieved is too low for polarising substantial volumes with reasonable optical intensity (even in optimized configurations)—over 10,000 seconds for polarising a microliter of fluid. Additionally, the presented scheme does not involve a coherent transfer of the polarisation from the NV centres to the nuclear spins—the main reason for the slow polarisation rate achieved.
The above analysis is also discussed in WO 2014/165845 A1. Three classes of motifs for the transfer of polarisation from a diamond to target nuclei external to the diamond are contemplated. Transfers governed by direct interactions between NV centres and external nuclei, transfers that utilize 13C nuclei hyperpolarisation within optically pumped diamond, and transfers mediated by other paramagnetic centres near the diamond's surfaces.
In “Dressed-State Polarization Transfer between Bright & Dark Spins in Diamond”, Physical Review Letters (2013), C Belthangady et al report the polarisation of electron spins in a nitrogen vacancy centre in diamond by optical pumping. The polarisation of the nitrogen centre electron spins can then be transferred to substitutional nitrogen electron spins by applying electromagnetic fields analogous to the Hartmann-Hahn matching condition. In this publication, only electron spins inside the diamond are polarised.
In “Sensitive magnetic control of ensemble nuclear spin hyperpolarization in diamond”, Nature Communication (2013), H-J Wang et al show polarisation of nuclear spins in contact interaction with a nitrogen vacancy colour centre in a diamond using the ground state level anti-crossing of the centre. The method shown was only relevant for nuclear spins inside the diamond, and not to external molecules.
In magnetic resonance applications, it is desirable to reach a higher degree of polarisation of the nuclear spins in the external molecules, in a higher volume and more quickly, than has hitherto been accomplished.